1. Field of Invention
The present invention relates generally offshore drilling and production systems, which are employed, e.g., for drilling and producing subsea oil or gas wells. In particular, the invention relates to systems for guiding and positioning risers on offshore drilling platforms by restricting lateral movement of the risers.
2. Description of Related Art
In one type of offshore system, a riser is connected between a subsea wellhead located on the sea floor and a drilling platform floating on the surface of the sea. In general, a riser is large diameter pipe used, e.g., to guide a drill string from the platform to the subsea wellhead and to provide a conduit through which drilling fluid may be circulated. Often, there is relative motion between the riser and the drilling platform since the subsea wellhead is not in exact alignment with the drilling platform, and since the drilling platform is subject to movement from wind and waves, while the riser is generally held stationary at the subsea wellhead.
It is important that relative movement between the platform and the riser, be limited to facilitate production and drilling operations, and to maintain clearance between the riser and other platform equipment. Throughout drilling and production operations, various pieces of equipment must be attached and detached from the riser. Thus movement of the platform relative to the riser may cause damage to the riser, the equipment attached to the riser, and the surrounding platform and platform equipment. Further, relative riser movement complicates the alignment and coupling of equipment which must be attached and detached from the riser.
Centralizing devices have been provided that constrain the lateral position of a riser relative to the platform. Some of these devices include rollers positioned circumferentially around the riser to guide longitudinal or vertical movements of the riser to facilitate drilling and production processes. Many of these centralizing devices, however, do not readily accommodate the unpredictable motion caused by waves, wind and other natural forces. Excessive loading can from these natural forces can cause the centralizing devices to prematurely wear and malfunction.